1. Field of the Invention
The present application is for an improved filter for transmitting electromagnetic radiation with high efficiency in an excitation frequency band (300-475 nm) and for reflecting electromagnetic radiation in higher bands (greater than 475 nm). The higher bands comprise the visible emission band and the infrared band. The improved filter can be prepared by applying an infrared hot-mirror coating to a dichroic filter or the infrared hot-mirror coating can be applied to or used in conjunction with a blue filter.
2. Description of the Related Art
Leak detection, materials detection and qualitative non-destructive testing are well suited to techniques employing fluorescence detection. These techniques rely upon the unique physical property of various materials to fluoresce when excited by certain wavelengths of visible or ultraviolet (UV) light.
It is a well-known phenomenon that electromagnetic energy within the near ultraviolet spectrum of approximately 315 to 400 nanometer wavelengths produces fluorescence in certain materials. That is, the fluorescent materials absorb radiated energy at the near UV or blue wavelengths and re-radiate or emit it at a longer wavelength in the visible spectrum. Thus, when fluorescent material absorbs electromagnetic energy in a specific excitation frequency band in a specific wavelength range, the material can emit electromagnetic energy in a characteristic fluorescent emission frequency band within the visible light spectrum. This phenomenon has enabled inspection and detection techniques in which fluorescent dyes, inks or pigments are illuminated by lamps selectively filtered to emit only ultraviolet radiation (invisible to the human eye) and then re-radiate with a high luminescence in the visible spectrum.
For example, the slow leakage of refrigerant from an air conditioning system is difficult to locate by any other means. The reason for the difficulty is because the refrigerant escapes as an invisible gas at such a low rate and rapid diffusion that the concentration of refrigerant in air near the leak site is difficult to differentiate from that surrounding any other location along the system circulation lines. However, by adding into the circulating system a small amount of fluorescent dye that is soluble in the refrigerant, the dye is carried out of the system with the refrigerant and glows brightly at the leak site when the area is swept with a UV lamp.
A similar procedure can be used to locate leaks of other fluids, such as lubricants, oils, fuels, heat transfer fluids or hydraulic fluids. Other UV inspection techniques use fluorescent dyes or paint to detect fissures or stress cracks in structural members.
Inspection lamps employ high intensity light sources operating at high temperatures to generate a sufficient photon flux for detection applications and utilize filters to absorb the undesirable wavelengths. A black light filter can be used but the filter is very restrictive and allows only UV wavelengths to be transmitted while all of the remaining wavelengths are absorbed. These filters typically have a transmission efficiency of 50-70% for the UV wavelengths (320-380 nm). To compensate for the limited transmission efficiency, the power of the lamps is very high in wattage and therefore heat producing. These lamps are usually 75-200 watts. The fluorescent dyes used in this system typically have maximum excitation in the range of 320-380 nm.
Some newer dyes respond well to higher wavelengths of light in the visible violet and blue range in addition to the invisible UV range (340-440 nm). With these dyes, improved photographic-type blue filters are used with smaller, low wattage lamps. These blue filters work well in lamps of 50 watts or less. At 50 watts, the lamps do not produce as much heat and because the blue filter allows some visible light to be transmitted, the dyes are still acceptably excited. In most cases, the lamps using these blue filters are also sold with special glasses (blue blocker glasses) that block the visible blue spectrum light transmitted through the blue filters. These blue filters assist the operator in finding the leaks and seeing the dye reaction to UV, blue and violet light. In addition, these blue filters are much more prone to temperature damage and cracking than the black light filters. However, the transmission efficiency is greater by about 10% as compared to that for the black light filters. Also, the blue filters and the blue blocker glasses provide more excitation of the dye in the visible range.
Newer improved filters have been developed by applying a dielectric coating, that does not effect the visible and lower spectrum of light transmission, to a piece of glass. Such filters are referred to as dielectric or dichroic filters. These terms are interchangeable. Dielectric refers to the process used, and dichroic is the type of coating applied, also known as thin-film coating. For example, dichroic filters with a dielectric coating have been developed in the entertainment industry and have high levels of transmission. The dichroic filter with a dielectric coating allows UV, blue and IR wavelengths to be transmitted while most visible wavelengths are blocked. Thus, this type of filter does not absorb the IR heat and has a transmission efficiency of over 90% for the desired wavelengths. These advantages allow users to reduce the size and wattage of the detection lamps.
For example, U.S. Pat. No. 5,905,268 (Garcia et al.) discloses an improved dichroic filter adapted to transmit electromagnetic radiation in the excitation frequency band (300-420 nm) and in the infrared and longer wavelength region (greater than 700 nm) and to reflect electromagnetic radiation in the fluorescent emission frequency band (420-700 nm).
The present application discloses an improved filter for transmitting electromagnetic radiation with high efficiency in the excitation frequency band (300-475 nm) and for reflecting electromagnetic radiation in the higher bands (greater than 475 nm) . The higher bands comprise the visible emission band (the fluorescent emission frequency band) and the infrared band. one major advantage of this improved filter is that it will allow manufactures to design higher output lamps using the blue filter and dichroic technology without damaging the filter, while allowing greater UV output and thus, improved effectiveness for the user in detecting fluorescence.
The present application discloses a filter for an ultraviolet lamp used for fluorescence detection. The filter comprises a filter coated with an infrared hot-mirror dichroic coating and adapted to transmit electromagnetic radiation in an excitation frequency band between 300 and 475 nm and for reflecting electromagnetic radiation in bands greater than 475 nm.
In a preferred embodiment, the filter is a blue filter. In another preferred embodiment, the filter is a dichroic filter. In a more preferred embodiment, the transmittance of electromagnetic radiation between 375 to 425 nm is greater than 90% for the infrared hot-mirror coated dichroic filter.
The present application also discloses a method for detecting leaks in an air-conditioning system. The method comprises adding a fluorescent dye to a fluid; placing the fluid into an air-conditioning system; examining the air-conditioning system with a fluorescent lamp equipped with a filter, where the filter comprises a filter coated with an infrared hot-mirror dichroic coating and adapted to transmit electromagnetic radiation in an excitation frequency band between 300 and 475 nm and for reflecting electromagnetic radiation in bands greater than 475 nm; and detecting any leaks in the air-conditioning system.
In a preferred embodiment of the method, the filter is a blue filter. In another preferred embodiment, the filter is a dichroic filter. In a more preferred embodiment, the transmittance of electromagnetic radiation between 375 to 425 nm is greater than 90% for the infrared hot-mirror coated dichroic filter.